(zilch lang go vfs): actually handle empty directories

In cases where a directory only contains files which are filtered out,
the VFS would break on importing the directory. This fixes that.

lang/go/src/vfs.sld

@@ -109,7 +109,7 @@
             (set! dirmap (mapping-update!/default dirmap dir (lambda (v) (cons (cons fname (zsymlink contents)) v)) '()))))
         (vfs-contents vfs))
       (define (read-dir dirname)
-        (define contents (mapping-ref dirmap dirname))
+        (define contents (mapping-ref/default dirmap dirname '()))
         (for-each
           (lambda (pair)
             (when (eq? (cdr pair) 'directory)